Ice cream evolved from iced beverages and fruit ices in early medieval times, some of which contained milk or cream. They were cooled with ice or snow containing salt and were quite popular in Europe in the mid 18th century. The first hard ice cream in the United States was produced in household machines and not until mid 19th century when milk dealers produced and sold ice cream as a luxury confectionery was it commercially sold. The hard and soft ice cream industry today utilizes approximately 9% of the total U.S. milk production in their products. Packaged hard ice cream has approximately a 100% overrun, which means the liquid mix has increased its volume 100%, and is served in a temperature range between 5.degree. F. and 10.degree. F. Soft ice cream has lesser overrun in the range of 50% and a serving temperature range of 20.degree. F. to 22.degree. F.
The first soft-serve machines were developed in the 1930's which were of the batch type and produced a batch of soft ice cream which was later hand-dipped since these earlier freezers were not specifically designed for serving the product directly from the machine. Since the soft-serve product was served at a much warmer temperature, the required content of butterfat and sugar was much less since the taste buds of the consumer were more sensitive at that warmer temperature. Hard ice cream contains approximately 10% butterfat, while soft-serve is in the range from 2 to 4%. Later-developed soft serve machines generally included a freezing cylinder with a rotary-dasher assembly. The dasher assembly included a scraper blade for removing the frozen product from the inside surface of the freezing chamber as it forms and an auger-shaped dasher blade which not only beats air into the mix but also forces the product to the dispensing end of the freezing chamber in a pumping action. Positioned above the freezing chamber is a reservoir containing a suitable mix in liquid form which when frozen and aerated sufficiently will yield the desired flavor and texture. The beating or whipping action of the dasher also thoroughly mixes the product with that being frozen on the surface of the freezer chamber to maintain a uniform temperature and texture throughout the chamber.
The freezing chamber and liquid product reservoir are typically connected in some manner so as to permit a suitable amount of new liquid product to move from the reservoir to the freezing chamber at the same time as frozen product is being dispensed from the freezing cylinder. In machines of the type which the present invention applies, the freezing chamber and reservoir are connected by an overrun control device which controls the amount of liquid mix and air which is drawn into the freezing cylinder. The common failing of prior gravity fed overrun control devices has been their inability to obtain and retain the proper "overrun" in the dispensed product.
When the frozen product is not being dispensed from the freezing cylinder, the machine must periodically cycle on and off so as to correctly maintain the temperature and texture of the frozen product in the freezing chamber. When the machine cycles on, the beater or dasher also runs in order to keep the frozen product cold and mixed so that it is ready for dispensing. As the machine remains in this "standby" condition for any extended period of time, the resultant product when finally dispensed will not contain as much air as it should and the result will lack proper taste and appear grainy. Typically, machines of this type will maintain an "overrun" in a range between 30% and 40% which is much less than the optimum 50% overrun condition. Various solutions have been suggested for holding overrun in the standby condition including the use of pumps for introducing pressurized or compressed air into the freezing chamber. Examples of machines using an external air pressure device are to be found in U.S. Pat. Nos. 3,196,633, 3,147,601 and 2,565,121. The disadvantages of the addition of such elaborate pumping devices is the increased energy which is required to run the machines, problems of mechanical failure and the difficulty in disassembling and cleaning.
The present invention adapts to a machine of the type referred to as gravity fed or non-mechanical which depend upon differences between pressure inside the freezing cylinder and atmospheric pressure above the mix in the mix tank. When product is drawn from the machine, pressure inside the cylinder is reduced and the mix is automatically metered into the freezing cylinder until a stable level is reached with no mechanical valves between the freezing cylinder and the liquid mix in the elevated reservoir.